Semiconductor switch

ABSTRACT

A semiconductor switch comprising a PNPN switch which has a four-layered PNPN structure with at least three PN-junctions, switching means for electrically short-circuiting a path between a P-type base and an N-type emitter of the PNPN switch when a transient voltage is applied between a P-type emitter and the N-type emitter of the PNPN switch, and drive means for driving the switching means, the drive means driving the switching means so that the path between the P-type base and the N-type emitter may be electrically short-circuited for a fixed time even after termination of the transient state, whereby any improper ignition of the PNPN switch is prevented even after the termination of the transient state.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a semiconductor switch which has afour-layered structure of a P-type emitter, an N-type base, a P-typebase and an N-type emitter and which has at least three PN-junctions.

Hereinbelow, the semiconductor switch having the PNPN structure shall besimply termed "PNPN switch."

The PNPN switch has the disadvantage that, even in the cut-off state,the switch is improperly ignited when an abrupt forward voltage isapplied between the P-type emitter (anode terminal) and the N-typeemitter thereof. Such a phenomenon is called the "dv/dt effect" or "rateeffect." In order to avoid the phenomenon, some proposals haveheretofore been made. The withstand strength up to which the PNPN switchis not improperly ignited by a transient voltage (dv/dt) is generallytermed the "dv/dt bearing capacity."

2. Description of the Prior Art

As proposals for enhancing the dv/dt bearing capacity, U.S. Pat. No.3,609,413 and U.S. Pat. No. 4,015,143 have been known. In both theproposals, a transistor is connected between the P-type base and N-typeemitter of a PNPN switch, and only when an abrupt forward voltage isapplied between the P-type emitter and N-type emitter of the PNPNswitch, the transistor is turned "on" to electrically short-circuit thepath between the P-type base and the N-type emitter, whereby anyimproper ignition is prevented.

According to these methods, certainly the dv/dt bearing capacity duringthe period during which the abrupt transient voltage is applied isenhanced. In some cases, however, an erroneous or improper ignitionoccurs after termination of the transient voltage.

This phenomenon is such that, when the short-circuiting transistor turns"off" at the time of the termination of the transient voltage, thevoltage between the P-type base and N-type emitter of the PNPN switchbecomes higher than the activating voltage of the PNPN switch, with theresult that the switch is improperly ignited.

A factor for the occurrence of the improper ignition is the recoverytime constant of the PNPN switch. When the recovery time constant isgreat, the erroneous ignition is prone to take place.

The recovery time constant is determined by the integration structure ofthe PNPN switches. In general, it increases as the density ofintegration is raised. Accordingly, it has heretofore been limited toenhance the dv/dt bearing capacity and to simultaneously raise thedensity of integration.

SUMMARY OF THE INVENTION

The first object of this invention is to provide a semicondutor switchwhich prevents any improper ignition from occurring after termination ofa transient voltage.

The second object of this invention is to provide a semiconductor switchwhich has a high dv/dt bearing capacity and a high density ofintegration.

The third object of this invention is to provide a semiconductor switchwhich has its cost lowered by making the density of integration high.

The first feature of this invention consists in disposing drive meansfor short-circuiting switching means connected between the P-type baseand N-type emitter of a PNPN switch, for a fixed time even aftertermination of a transient voltage.

The second feature of this invention is that the drive means isconstructed of an electrostatic capacitive element and a pulse widthexpand circuit which are disposed between the P-type emitter of the PNPNswitch and a control terminal of the switching means.

The third feature of this invention is that the drive means isconstructed of a PN-junction device of an inverse junction and a pulsewidth expand circuit which are connected in series between the N-typebase of the PNPN switch and a control terminal of the switching means.

The fourth feature of this invention is that the drive means isconstructed of a PNP transistor which is disposed between the N-typebase of the PNPN switch and a control terminal of the switching means,and a PN-junction device of an inverse junction which is connectedbetween the emitter and base of the PNP transistor.

BRIEF DESCRIPTION OF THE DRAWING

FIGS. 1, 3 and 4 are connection diagrams each showing an embodiment ofthis invention.

FIG. 2 is a waveform diagram for explaining the operation of theembodiment shown in FIG. 1.

FIGS. 5 and 6 are circuit diagrams respectively showing concreteembodiments of pulse width expand circuits in FIGS. 1 and 3.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows an embodiment of this invention. Fundamentally, it is madeup of a PNPN switch 10, switching means 20, an electrostatic capacitiveelement 30 and a pulse width expand circuit 40.

The electrostatic capacitive element 30 and the pulse width expandcircuit 40 constitute drive means to be described later.

The PNPN switch 10 has a four-layered structure consisting of a P-typeemitter (P₁), an N-type base (N₁), a P-type base (P₂) and an N-typeemitter (N₂). The P-type emitter (P₁) is provided with an anode terminalA, the N-type emitter (N₂) is provided with a cathode terminal K, andthe P-type base (P₂) is provided with a gate terminal G. By impressing agate signal on the gate terminal G, the PNPN switch 10 is ignited.

The switching means 20 is concretely illustrated as an NPN transistorhere. It has an input terminal 21, an output terminal 22 and a controlterminal 23. The input terminal 21 is connected to the P-type base (P₂)of the PNPN switch 10, while the output terminal 22 is connected to theN-type emitter (N₂) of the PNPN switch 10.

The control terminal 23 of the switching means 20 is connected to anoutput terminal 42 of the pulse width expand circuit 40. By a drivingsignal from the pulse width expand circuit 40, the switching means 20 isoperated to short-circuit the path between the P-type base (P₂) andN-type emitter (N₂) of the PNPN switch 10.

The electrostatic capacitive element 30 and the pulse width expandcircuit 40 form the drive means for operating the switching means 20.The drive means detects the transient state in which the voltage betweenthe P-type emitter (anode A) and N-type emitter (cathode K) of the PNPNswitch 10 rises abruptly, and it delivers the driving signal to thecontrol terminal 23 of the switching means 20 during a period which islonger than the period of the transient state.

Let it now be supposed that a voltage V_(AK) having a ramp functionwhich rises at a time t₀ and reaches a predetermined value at a time t₁as indicated in FIG. 2 has been applied to the anode A of the PNPNswitch 10.

In this case, the transient voltage V_(AK) is differentiated by acapacitor 31, and the differential current I_(B) is entered into thepulse width expand circuit 40.

The pulse width expand circuit 40 sustains the current I_(B) for drivingthe switching means 20, for a predetermined time T_(H) after the time t₁at which the rise of the transient voltage terminates. Thus, anyimproper or erroneous ignition of the PNPN switch after the terminationof the rise of the transient voltage is prevented.

The operation will be explained more concretely.

T_(P) is let to denote the period of time which is taken after the timet₁ of the termination of the rise of the transient voltage V_(AK) inorder that the terminal voltage V_(G) of the gate G at the time when theswitching means 20 is not connected may lower down to the activationvoltage V_(B) of the PNPN switch (as indicated by a broken line in thefigure). T_(S) is let to denote the period of time which is taken untilthe switching means 20 turns "off" and then the gate G terminal voltagerises again and reaches its peak point. Heretofore, it has been theactual circumstances that when the period T_(S) becomes shorter than theperiod T_(p), a voltage equal to or greater than the activation voltageV_(B) appears at the gate G terminal of the PNPN switch 10 even afterthe termination of the rise of the transient voltage, so that the switchis improperly ignited.

In contrast, in the semiconductor switch shown in FIG. 1, even in casewhere the period of time T_(P) which is taken until the gate terminalvoltage V_(G) drops down to the activating voltage V_(B) owing to therecovery time constant τ_(P) of the PNPN switch 10 is longer than theperiod of time T_(SO) which is taken until the switching means 20 turns"off" after the time t₁ of the termination of the rise of the transientvoltage (even in case where T_(P) >T_(S) ≈T_(SO)), the condition ofT_(P) <T_(S) ≈T_(SO) +T_(H) holds because the current I_(B) for drivingthe switching means 20 is sustained for the predetermined period of timeT_(H) by the pulse width expand circuit 40. Therefore, even after thetermination of the rise of the transient voltage, the erroneous ignitioncan be perfectly prevented.

A resistor 70 in FIG. 1 has heretofore been employed. It is disposedbetween the P-type base (P₂) and N-type emitter (N₂) of the PNPN switch10, and its magnitude has influence on the gate sensitivity and thedv/dt bearing capacity. In general, when the resistance of the resistor70 is made high, the gate sensitivity improves, but the dv/dt bearingcapacity lowers. Conversely, when the resistance is made low, the gatesensitivity worsens, but the dv/dt bearing capacity rises.

A resistor 60 is a load of the PNPN switch 10. A battery 50 is a powersource of the pulse width expand circuit 40. Terminals 41, 42, 43 and 44are external terminals of the pulse width expand circuit 40.

FIG. 3 shows the second embodiment of this invention. Here, theswitching means 20 and the pulse width expand circuit 40 are the same asin FIG. 1. As the electrostatic capacitive element 30, a PN-inversejunction electrostatic capacity made of a diode 32 is utilized. Inaddition, one end of the electrostatic capacitive element 30 isconnected to the N-type base (N₁) of the PNPN switch 10.

Also in the semiconductor switch in FIG. 3, when a transient voltage isapplied to the anode A, a differential current flows from the N-typebase (N₁) of the PNPN switch 10 through the inverse junctionelectrostatic capacity of the diode 32 to the pulse width expand circuit40, and it is sustained for a predetermined time by the pulse widthexpand circuit 40, so that the same effect as in the embodiment of FIG.1 can be expected.

FIG. 4 shows the third embodiment of this invention. In this embodiment,a PNP transistor 80 which consists of a P-type collector (P₃), an N-typebase (N₃) and a P-type emitter (P₄) is connected between the N-type base(N₁) of the PNPN switch 10 and the control terminal 23 of the switchingmeans 20. Further, a PN-junction device 90 which consists of layers P₅and N₅ and which has an electrostatic capacity is disposed between thebase (N₃) and emitter (P₄) of the PNP transistor 80.

When FIG. 4 is compared with FIG. 1, a portion corresponding to theelectrostatic capacitive element 30 in FIG. 1 is the junctionelectrostatic capacity of the layers N₃ and P₃ of the PNP transistor 80in FIG. 4, and a portion corresponding to the pulse width expand circuit40 is the PN-junction device 90.

In FIG. 1, the electrostatic capacitive element 30 and the pulse widthexpand circuit 40 constitute the drive means for the switching means 20.In FIG. 4, the PNP transistor 80 and the PN-junction device 90 form thedrive means.

In operation, when an abrupt transient voltage is applied to the anode Aof the PNPN switch 10, a current changing the junction between the base(N₃) and collector (P₃) of the transistor 80 flows to the controlterminal 23 of the switching means 20 in the process of the rise of thetransient voltage (during a period t_(0-t) ₁ in FIG. 2). Thus, the pathbetween the gate G and cathode K of the PNPN switch 10 is suppressed toa low resistance, and the erroneous or improper ignition is prevented.The charging current of the base (N₃) - collector (P₃) junction at thistime consists of a current which flows through the emitter (P₄) of thetransistor 80, and a current which charges the electrostatic capacity ofthe layer N₅ - layer P₅ junction of the PN-junction device 90.Therefore, charges are stored in the inverse junction electrostaticcapacity of the PN-junction device 90.

After completion of the rise of the transient voltage (after t₁indicated in FIG. 2), the charging current based on the transientvoltage is not received from the anode A. Since, however, the chargesstored in the inverse junction of the PN-junction device 90 aredischarged through the emitter (P₄) and base (N₃) of the transistor 80,the equivalent recovery time of the transistor 80 becomes long, and thebase current of the transistor being the switching means 20 can besustained. In this manner, when the transient voltage is applied to thePNPN switch 10, the switching means 20 can be operated, not only in theprocess of the rise of the transient voltage, but also for a periodlonger than the recovery time T_(P) of the PNPN switch (the period oftime taken until the gate voltage of the PNPN switch becomes less thanthe activation voltage V_(B) ≈0.7 V) even after the termination of therise of the transient voltage. The path between the gate G and cathode Kof the PNPN switch 10 can accordingly be short-circuited during such aperiod.

FIG. 5 shows a concrete example of the pulse width expand circuit 40 inFIG. 1 or FIG. 3. This example is a monostable multivibrator which ismade up of transistors 40-1 and 40-2, capacitors 40-3 and 40-4, andresistors 40-5 to 40-9. When a current pulse (the current from theelectrostatic capacitive element 30) is impressed on an input terminal41 of the monostable multivibrator 40, a voltage of a certain pulsewidth determined by the time constant of the monostable multivibrator 40is generated, and a current for driving the switching means 20 issupplied from an output terminal 42 through the resistor 40-9.

FIG. 6 shows another concrete emnbodiment of the pulse width expandcircuit 40. It is made up of a transistor 40-10, a capacitor 40-11 and aresistor 40-12. When a current pulse is impressed on an input terminal41, a supply voltage (the voltage of the battery 50 indicated in FIG. 1)applied to the collector of the transistor 40-10 is given to thecapacitor 40-11. After the input current pulse has vanished, a currentfor driving the switching means 20 is supplied from an output terminal42 according to the time constant between the resistor 40-12 and thecapacitor 40-11. The current for driving the switching means 20 canaccordingly be sustained by appropriately selecting the values of theresistor 40-12 and the capacitor 40-11.

In the foregoing embodiments of this invention, the current-drivenswitch of the PNPN four-layered structure is employed as thesemiconductor switch. Also in case of employing a photo-driven switch, ahigh dv/dt bearing capacity as in the previous embodiments is attained,and a semiconductor switch having a high density of integration can beprovided.

We claim:
 1. A semiconductor switch comprising:(a) a PNPN switch whichhas a four-layered structure consisting of a P-type emitter, an N-typebase, a P-type base and an N-type emitter and which has at least threePN-junctions, (b) switching means which has an input terminal, an outputterminal and a control terminal,said input terminal and said outputterminal of said switching means being respectively connected to saidP-type base and said N-type emitter of said PNPN switch, a path betweensaid P-type base and said N-type emitter of said PNPN switch beingelectrically short-circuited by a driving signal impressed on saidcontrol terminal, and (c) drive means for delivering said driving signalto said control terminal of said switching means,said drive meansdetecting a transient state in which a voltage between said P-typeemitter and said N-type emitter of said PNPN switch rises abruptly, anddelivering said driving signal to said control terminal of saidswitching means during a period which is longer than a period of saidtransient state.
 2. A semiconductor switch according to claim 1, whereinsaid drive means is constructed of an electrostatic capacitive elementand a pulse width expand circuit which are connected in series betweensaid P-type emitter of said PNPN switch and said control terminal ofsaid switching means.
 3. A semiconductor switch according to claim 1,wherein said drive means is constructed of a PN-junction device of aninverse junction and a pulse width expand circuit which are connected inseries between said N-type base of said PNPN switch and said controlterminal of said switching means.
 4. A semiconductor switch according toclaim 1, wherein said drive means is constructed of a PNP transistorwhich is connected between said N-type base of said PNPN switch and saidcontrol terminal of said switching means and which has an emitterterminal and a collector terminal thereof connected to said N-type baseand said control terminal respectively, and a PN-junction device whichis connected with its N-layer lying on the emitter terminal side of saidPNP transistor and with its P-layer lying on the base terminal side ofsaid PNP transistor.
 5. A semiconductor switch according to claim 2,wherein said pulse width expand circuit is a one-shot multivibratorwhich is driven by a transient current entered through saidelectrostatic capacitive element and which delivers a prescribed pulsewidth.
 6. A semiconductor switch according to claim 2, wherein saidpulse width expand circuit is constructed of an NPN transistor whosebase terminal is connected to said electrostatic capacitive element,whose collector terminal is connected to a driving power source andwhose emitter terminal is connected to a capacitor, and a resistor whichis connected between said emitter terminal of said PNP transistor andsaid control terminal of said switching means.
 7. A semiconductor switchaccording to claim 2, wherein said pulse width expand circuit is aone-shot multivibrator which is driven by a current entered through aPN-junction device of an inverse junction and which delivers aprescribed pulse width.
 8. A semiconductor switch according to claim 3,wherein said pulse width expand circuit is constructed of an NPNtransistor whose base terminal is connected to a P-layer of saidPN-junction device of the inverse junction, whose collector terminal isconnected to a driving power source and whose emitter terminal isconnected to a capacitor, and a resistor which is connected between saidemitter terminal of said NPN transistor and said control terminal ofsaid switching means.